Convective Initiation by Topographically Induced Convergence Forcing over the Dabie Mountains on 24 June 2010

The initiation of convective cells in the late morning of 24 June 2010 along the eastward extending ridge of the Dabie Mountains in the Anhui region, China, is studied through numerical simulations that include local data assimilation. A primary convergence line is found over the ridge of the Dabie Mountains, and along the ridge line several locally enhanced convergence centers preferentially initiate convection. Three processes responsible for creating the overall convergence pattern are identified. First, thermally-driven upslope winds induce convergence zones over the main mountain peaks along the ridge, which are shifted slightly downwind in location by the moderate low-level easterly flow found on the north side of a Mei-yu front. Second, flows around the main mountain peaks along the ridge create further convergence on the lee side of the peaks. Third, upslope winds develop along the roughly north-south oriented valleys on both sides of the ridge due to thermal and dynamic channeling effects, and create additional convergence between the peaks along the ridge. The superposition of the above convergence features creates the primary convergence line along the ridge line of the Dabie Mountains. Locally enhanced convergence centers on the primary line cause the initiation of the first convection cells along the ridge. These conclusions are supported by two sensitivity experiments in which the environmental wind （dynamic forcing） or radiative and land surface thermal forcing are removed, respectively. Overall, the thermal forcing effects are stronger than dynamic forcing given the relatively weak environmental flow.

A Modeling Study on the Mechanism of Convective Initiation of a Squall Line over Northeastern China

High-resolution numerical simulation results of a squall line initiated along a convergence zone in northeast China on 26 June 2014 were presented in this study.The simulation was performed by a convection-permitting model with coarse and fine grids of 4 and 1.33 km,respectively,and the simulation results were validated against the observation.Results showed that the simulation adequately reproduced the life cycle of the squall line,which allowed detailed investigation of the mechanism of convective initiation in this case.The synoptic condition was favorable for convective initiation and the convection was triggered in a convergence zone,where a branch of dry and cold air and a branch of moist and warm air collided.The water vapor flux divergence was inhomogeneous and some cores of water vapor convergence existed in the convergence zone.These cores were the spots where water vapor converged intensely and the air there was forced to rise,creating favorable spots where the convection was initially triggered.A series of quasi-equally spaced vortices near the surface,which themselves were the result of horizontal shear instability,were accountable for the inhomogeneity of the surface water vapor flux divergence.These vortices rotated the moist air into their north and dry air into their south,thus creating more favorable spots for convective initiation in their north.After initiation,the updraft turned the horizontal vorticity into vertical vorticity in the mid-level.The vortices near the surface collaborated with the vorticity maxima in the mid-level and enhanced the development of convection by providing water vapor.

Identification of Forcing Mechanisms of Convective Initiation over Mountains through High-Resolution Numerical Simulations

Convection and its ensuing severe weather, such as heavy rainfall, hail, tornado, and high wind, have significant im- pacts on our society and economy （e.g., Cao et al., 2004; Fritsch and Carbone, 2004; Verbout et al., 2006; Ashley and Black, 2008; Cao, 2008; Cao and Ma, 2009; Zhang et al., 2014）. Due to its localized and transient nature, the initiation of convection or convective initiation remains one of the least understood aspects of convection in the scientific communi- ties, and it is a significant challenge to accurately predict the exact timing and location of convective initiation （e.g., Cai et al., 2006; Wilson and Roberts, 2006; Xue and Martin, 2006; Cao and Zhang, 2016）.

A Case Study of the Initiation of Parallel Convective Lines Back-Building from the South Side of a Mei-yu Front over Complex Terrain

Parallel back-building convective lines are often observed extending to the southwest of some mesoscale convective systems(MCSs)embedded in the mei-yu front in China.The convective lines with echo training behavior can quickly develop into a stronger convective group of echoes,resulting in locally heavy rainfall within the mei-yu front rainband.The initiation mechanism of the back-building convective lines is still unclear and is studied based on high-resolution numerical simulation of a case that occurred during 27−28 June 2013.In the present case,the new convection along the convective lines was found to be forced by nonuniform interaction between the cold outflow associated with the mei-yu front MCSs and the warm southerly airflow on the south side of the mei-yu front,which both are modified by local terrain.The mei-yu front MCSs evolved from the western to the eastern side of a basin surrounded by several mesoscale mountains and induced cold outflow centered over the eastern part of the basin.The strong southwest airflow ahead of the mei-yu front passed the Nanling Mountains and impacted the cold outflow within the basin.The nonuniform interaction led to the first stage of parallel convective line formation,in which the low mountains along the boundary of the two airflows enhanced the heterogeneity of their interaction.Subsequently,the convective group quickly developed from the first stage convective lines resulted in apparent precipitation cooling that enhanced the cold outflow and made the cold outflow a sharp southward windshift.The enhanced cold outflow pushed the warm southerly airflow southward and impacted the mountains on the southeast side of the basin,where the roughly parallel mountain valleys or gaps play a controlling role in a second stage formation of parallel convective lines.

Statistical Characteristics of Convective Initiation in the Beijing-Tianjin Region Revealed by Six-Year Radar Data

Characteristics of convective initiation（CI） in the Beijing-Tianjin region during the warm season of 2008-2013 are examined.A total of 38877 CI cases are identified by a thunderstorm identification,tracking,analysis,and nowcasting algorithm.CI cases are evaluated in the context of associated terrain,weather systems,and land cover properties.The spatial distribution of all CI cases shows that there are dense CI activities around the 200-m elevation,which means that convective storms are more easily triggered over foothills.From 1500-1800 to 0300-0600 BT（Beijing Time）,the high-occurrence CI region tends to propagate southeastward（i.e.,from mountains to plains,then to ocean）.Among the four local weather systems,the Mongolian cold vortex has the highest CI frequency while the after-trough system has the lowest CI frequency.For the land cover relationships with CI,the urban land cover has the highest CI density and the forest-type land cover has the second highest CI density;these two types of land cover are more conducive to CI formation.

Numerical Simulations of a Florida Sea Breeze and Its Interactions with Associated Convection:Effects of Geophysical Representation and Model Resolution

The Florida peninsula in the USA has a frequent occurrence of sea breeze(SB)thunderstorms.In this study,the numerical simulation of a Florida SB and its associated convective initiation(CI)is simulated using the mesoscale community Weather Research and Forecasting(WRF)model in one-way nested domains at different horizontal resolutions.Results are compared with observations to examine the accuracy of model-simulated SB convection and factors that influence SB CI within the simulation.It is found that the WRF model can realistically reproduce the observed SB CI.Differences are found in the timing,location,and intensity of the convective cells at different domains with various spatial resolutions.With increasing spatial resolution,the simulation improvements are manifested mainly in the timing of CI and the orientation of the convection after the sea breeze front(SBF)merger into the squall line over the peninsula.Diagnoses indicate that accurate representation of geophysical variables(e.g.,coastline and bay shape,small lakes measuring 10-30 km2),better resolved by the high resolution,play a significant role in improving the simulations.The geophysical variables,together with the high resolution,impact the location and timing of SB CI due to changes in low-level atmospheric convergence and surface sensible heating.More importantly,they enable Florida lakes(30 km2 and larger)to produce noticeable lake breezes(LBs)that collide with the SBFs to produce CI.Furthermore,they also help the model reproduce a stronger convective squall line caused by merging SBs,leading to more accurate locations of postfrontal convective systems.

Numerical Study on the Initiation of the Severe Convective Weather in Chongqing on 6 May 2010

As a follow-up of a previously published article on the synoptic background of the development of the severe convective weather that happened in Chongqing on 6 May 2010, this study further examines the initiation of the severe convective weather via a better high-resolution simulation with the Weather Research and Forecasting （WRF） model. It is found that the cold front approaching Chongqing from the northwest played a critical role in the initiation of the severe convective weather. As the cold front approached Chongqing, the low-to-mid level updrafts ahead of the front acted to increase the atmospheric lapse rate via the stretching effect, which in combination with the low-level diabatic heating induced by the sensible heat fluxes and infrared radiation emitted from the ground surface led to the continuous decrease of the low-level static stability and the increase of the convective available potential energy （CAPE） in Chongqing area. This provided necessary unstable energy for the development of deep moist convection. Furthermore, along with the reaching of a nearly east-west-oriented mesoscale convergence line from the southeast of Chongqing, the outflow right above the cold front began to interact with that above the mesoscale convergence line and induced distinct convergence at the altitude of approximately 1-2 km in the triangular area sandwiched by the cold front and the mesoscale convergence line. It is found that the updrafts associated with this convergence provided lifting necessary for the initiation of the severe convection. The sensitivity experiment without the terrain west of Chongqing indicates that the local topography did not play an important role in the initiation of this severe convective weather.

Convection Initiation of a Heavy Rainfall Event in the Coastal Metropolitan Region of Shanghai on the South Side of the Meiyu Front

Accurate prediction of the convection initiation(CI)in urban areas is still a challenge.A heavy rainfall event,missed by the 9-km regional operational modeling system,occurred in the coastal urban area of the Shanghai metropolitan region(SMR)in the late morning on 28 July 2020 on the warm side to the south of the Meiyu front.In this study,observational analyses and convection-permitting simulations with a resolution of 3 km were conducted to investigate the CI mechanism of this rainfall event.The results showed that the CI was due to the interaction of urban heat island(UHI),northwesterly outflows from the Meiyu front precipitation system(MFPS),and northeasterly sea winds.First,the UHI created a lifting condition producing adiabatic cooling and the vertical moisture transport in the urban region.Then,the mesolow generated by the UHI induced and enhanced local low-level convergence near the CI region and accelerated the northwesterly outflows and the northeasterly sea winds as they converged to the UHI.The convection was triggered as a result of the strengthened low-level convergence when the enhanced northwesterly outflows and northeasterly sea winds approached the updraft zone caused by the UHI center.Sensitivity experiments with either the urban area of the SMR removed or the MFPS suppressed further revealed that the enhancement of the low-level convergence was mainly contributed by the UHI.The outflows and sea winds transported cold and moist air to the CI region and partly offset the negative contribution of the urban drying effect to the low-level relative humidity to facilitate the development of the deep moist absolute unstable layer during the CI.In addition,the MFPS also contributed to the enhancement of the northeasterly sea winds by influencing the land–sea pressure contrast on the north of the SMR.